Process of manufacturing a master dot pattern for photoetching a graded-hole shadow mask

ABSTRACT

A process for the production of master dot patterns for photoetching a graded-hole shadow mask for use in a color picture tube including mounting a grading mask having its transparency concentrically, progressively decreasing from the center toward the periphery thereof between a light source and a photographic material and inserting a screen between the grading mask and the light source.

United States atent Noguchi 1 Feb. 29, 1972 [54] PROCESS OF MANUFACTURING A 2,972,931 2/1961 Roob ..355/7l MASTER DOT PATTERN FOR 2,145,427 1/1939 Morris, Jr ....95/64 PHOTQETCHING A GRADEILHOLE 3,152,900 10/1964 Kaus ...96/36.1 2,937,297 5/1960 Burdick .313/85.5 SHADOW MASK 3,109,117 10/1963 Kaplan ..3l3/86 [72] Inventor: Kinich Noguchi, Tokyo, Japan 3,146,368 8/ 1964 Fidre et al.. .96/36.1 [73] Assignee: Toppan Priming Co" UMTOkyO Japan 2,663,821 12/1953 Law ..3l3/85.5 22] il 22 19 9 OTHER PUBLICATIONS 21 APPL 333, 3 Anon, Construction of Tricolor Picture Tube," Electronics,

May, 1951, (pp. 86- 88) Related 115. Application Data 1 Primary Examiner-Norman G. Torchin [63] fggmuatlon m part of Ser. No. 387,724, Aug. 5, Assistant Examiner Edward C- Kimlin Attorney-Anthony A. O'Brien [52] U,S.Cl ..96/36.l,96/36.2, 96/116 [51 Int. Cl 1571 ABSTRACT (58] Field of Search ..96/35, 36, 36.1, 36.2, 27; A

I process for the production of master dot patterns for 313/85 92 95/18 22 5 photoetching a graded-hole shadow mask for use in a color 355/71 1 picture tube including mounting a grading mask having its transparency concentrically, progressively decreasing from [56] References Cited the center toward the periphery thereof between a light source UNITED STATES PATENTS and a photographic material and inserting a screen between the grading mask and the light source. 2,625,734 1/1953 Law ..96/35 Morrell ..3 13/85 12 Claims, 12 Drawing Figures PAIENTEDFEm I972 sum 2 or 3 d .2 x a Fig.8.

I00 Disfanco from disc cenfor 'PATENTEDFEBZWBYZY I I 8,645,734

sun-:1 3 or 3 Fig.9. 4 Fig.l0.

Dot Dwmofor Densiiy of grading mask 0 Density of grading mu k Dm 0 Distance from disc center Fig .11.

Densify LoqE o Expose value PROCESS OF MANUFACTURING A MASTER DOT PATTERN FOR IPIIOTOETCIIING A GRADED-IIOLE SHADOW MASK This application is a continuation-in-part of Ser. No. 387,724, filed 8-5-69.

This invention relates to the process of manufacturing a master dot pattern for photoetching a graded-hole shadow mask used in a color television picture tube and has particular reference to a grading mask used in the said process.

Existing shadow masks used for color picture tube are, in most cases, made of a thin sheet of steel or other metal which is provided with a number of regularly aligned pores to permit the transmission of an electron beam therethrough. More recent improvements in this type of shadow mask comprise .the provision of relatively large pores in the center of the effective mask area and reducing the size of concentrically aligned pores progressively toward the peripheral portion of the mask while eliminating a miss landing of electrons at the periphery of the screen of the tube. This is what is termed a graded-hole shadow mask which is manufactured by a photoetching process.

Whereas, it is a principal object of this invention to provide novel method and means of preparing a master dot pattern for photoetchin g of graded-hole shadow masks which is provided with a dot diameter larger at the center of the effective surface and progressively smaller toward the peripheral portion of the mask.

It is another object of the invention to provide a novel process of making a photoetching master dot pattern with the use of a grading mask adapted to control the intensity of exposure light so that the brightness of a picture is greater at the center and smaller toward the periphery of the mask.

It is a further object of the invention to provide a photoetching master dot pattern which has a suitable pore size distribution by varying the density of the grading mask.

It is still another object of the invention to provide simple, economical means of obtaining the above grading mask for use in the manufacture of photoetching master dot patterns.

These and other objects of the invention will become more apparent from the following description taken in connection with the accompanying drawings illustrative of the manner of preparing a master dot pattern for photoetching of gradedhole shadow masks according to the invention.

In the drawings:

FIG. 1 is a perspective view of a grading mask according to the present invention,

FIG. 2 is a perspective view of a grading mask using an optical wedge according to the present invention;

FIG. 3 is a perspective view of a grading mask using a plurality of sheets according to the present invention;

FIG. 4 is a perspective view of a grading mask in the form of a flattop cone according to the present invention;

FIG. 5 is a front view of a glass screen for photographing a shadow mask etching master dot pattern which has an angle of 60 crossover of line groups;

FIG. 6 schematically illustrates the arrangement of a device for photographing a graded-hole shadow mask master pattern using a process camera;

FIG. 7 schematically illustrates the arrangement of a device for photographing a graded-hole shadow mask master pattern using a printing frame;

FIG. 8 is a graph illustrating the relation between the distance from the center of the effective area of the master dot pattern for photoetching of graded-hole shadow masks to the periphery thereof and the dot diameter;

FIG. 9 is a graph illustrating the relation between the grading mask density and the screen dot diameter where the photographic material, time of exposure and development conditions are all held invariable for the screening of the grading mask placed before a light source in the production of a shadow mask pattern;

FIG. I graphically illustrates the density distribution of the grading mask obtained from the graph of FIG. 9;

FIG. 11 graphically illustrates the characteristic curve of a dry plate used in the manufacture of a grading mask; and

FIG. 12 schematically illustrates the requirements which determine the shape of an opening in a disc hereinafter referred to as sector-disc which is used in the manufacture of a grading mask.

Generally, a master dot pattern required for photoetching of shadow masks may be obtained by passing thereto a light through a screen having a transparent spot at each corner of an equilateral triangle. In such instance, the pattern may be subjected to screening through a lithographic glass screen or contact screen 3 having a rhombic opening 2 with an angle of 60 for crossing of line groups 1, as illustrated in FIG. 5, thereby obtaining dots of uniform size over the entire surface of the pattern.

In order to vary the dot size successively from the effective center area toward the periphery of the pattern, the present invention contemplates the use of a grading mask which may be placed in front of a screening light source to control the intensity of light therefrom.

The grading mask based on an entirely new concept of the invention comprises a suitable photographic material such as dry plate, film or the like which has a transmittance of rays high at the center and progressively lower toward the peripheral portion of the mask, such transmittance variation being determined by the distribution of the pore size required for the performance of a desired shadow mask.

As a result of extensive search for the best of the processes of producing the aforementioned grading mask, the present inventor discovered the following:

I. A disc member 11 having a transparent spot 11a and an opaque spot 11b, which is similar to a sector-disc as used in a sensitometer such as for example I-Iurter-Driffield sensitometer and Schiner sensitometer, is held in opposite relation to a relatively low contrast photographic material 12. While either the sector-disc 11 or the photographic material 12 is rotated about the center C of the sector-disc, light is passed from the disc onto the photographic material thereby obtaining a grading mask. The shape of the transparent spot 11a of the sectordisc 11 may be determined by the characteristic curve obtained from the tests conducted under the same conditions as in the manufacture of the grading mask. This process has the advantage that a density distribution is obtained with accuracy as desired, although the rotating center may be somewhat unstable.

2. As shown in FIG. 2, an opaque disc 13 is provided with a transparent spot 13a having a suitable angle of opening and with an optical wedge 13b having a predetermined density distribution and being superimposed upon said transparent spot. The opaque disc 13 is positioned in opposite relation to a relatively low contrast photographic material 14. Light is passed from the disc member 13 through the photographic material member 14 while either of the two members is rotated about an axis taken on the maximum density spot 15 of the optical wedge 13b. Density distribution of the optical wedge 13b may be determined by the characteristic curve obtained from tests carried out under exactly the same conditions as provided for the production of a grading mask. This process has the advantage that the angle of opening is formed with ease and the center of rotation is finished to a reasonable accuracy.

3. As illustrated in FIG. 3, there are provided a plurality of semitransparent sheets 16, 16', 16" 16' made of a synthetic resin film or paper having a uniform transmittance of light. They are provided with circular windows 17, I7, 17" I7, which are concentrically arranged but different in diameter, and are stacked together with the sheet 16 having the smallest window 17 lying lowermost of the stack followed by successively larger window sheets and with the sheet 16 having the largest window 17 lying on top of the stack. Alternatively, such pattern of apertures may be printed on a photographic material. In this manner, it is easy to fabricate a grading mask. However, this process is handicapped in that cut ends of the apertures in the sheets are noticeable and some difficulty is encountered in grading such sheets for proper light transmittance. I

4. The process illustrated in FIG. 4 involves a semitransparent block 18 of a synthetic resin or the like which has uniform transmittance of light. The block is carved in the form of a flattop cone which is thin at the center 18a and grows thicker progressively toward the periphery thereof. Alternatively, such pattern may be printed on a photographic material. According to this process, a good selection of transparent materials may be made matt on the surface to act as a lens thereby allowing the grading to vary as desired.

A detailed description follows of each of the above processes for the production of grading masks according to the invention.

PROCESS (l EXAMPLE a. Determining the density distribution of the grading mask.

Density distribution of the grading mask required for the preparation of a graded-hole shadow mask. pattern having a requisite screen dot size distribution may be obtained in the following manner.

In the photographing of a shadow mask pattern, the grading mask is placed before a light source and screened where the photographic material, time of exposure and development conditions are held invariable. In this manner, there may be established a certain relation between the grading mask density and the screen dot diameter. This may be graphically displayed as shown in FIG. 9, from which a desired density distribution of the grading mask may be readily obtained as shown in FIG. 10.

b. Determining the characteristic of a dry plate.

A characteristic curve for the dry plate to be used in the production of grading masks may be obtained as follows.

A dry plate is placed on a turntable immediately underneath an enlarging lens in a manner similar to the fabrication of a grading mask. To the dry plate is adhered a step wedge of which density is known. It is exposed to light for a suitable length of time and then subjected to development under certain conditions. By determining the density of the resulting dry plate in view of the known density of the step wedge, it is possible to obtain a characteristic curve prevailing under such development conditions. See FIG. 11, from which the exposure light intensity E required to obtaina desired density D may be readily determined.

c. Determining the shape of the opening in the sector-disc.

(See FIG. 12)

From the definition of exposure light intensity, there may be established an equation E=I-T where I is light intensity and Tis exposure time. If light transmittance is uniform over the entire area of the transparent opening in the sector-disc, the factor I remains constant, hence EaT. When light is applied while either of the sector-disc and the dry plate is being rotated, the time of exposure Tr at point 8 away from the center of the disc is proportional to an arc length Sr of the radius of the opening in the sector-disc. The arc length lr is further proportional to the angle Qr which points toward the center of the disc, so that there may be established a relation defined by EaQ; that is E=K-Q where K is an optional multiplier.

Accordingly, the sequence of fabricating a grading mask may be given as follows:

I. The value 1' is selected as often as required successively from the point 0.

2. The value D corresponding to each of the r values is obtained from FIG. 10.

3. The value log E corresponding to each of the D values is obtained from FIG. 11.

4. The value E is derived from log E thereby obtaining Q from the equation of E=KQ. In this instance, the value K determines the total size of a transparent opening.

Listed below are the points of plotting the coordinates to define the transparent opening of the disc.

Angle (x-axis Distance from disc center C With reference to a curve drawn by connecting the plotted dots, a base having a transparent spot and an opaque spot may be prepared and subjected to photographing with a dry plate thereby producing a sector-disc. This disc is carried in a negative holder on an enlarger. A turn table having a dry plate holder mounted thereon and being driven by a phonomotor is placed immediately below the enlarger lens. With the image of the sector-disc projected onto the dry plate, the center of the disc may be registered with the center of rotation of the turntable.

As a light source for the enlarger, there may be used a v., I50 w. enlarger light bulb. The lens may be specified for a focal distance of mm. and a diaphragm of f.22. While the turntable is rotated at a speed of 78 rpm, at duplicating 0150 dry plate is exposed to light for 20 seconds. The dry plate is developed with a specified developer to a low contrast.

Now, the manner in which photographing is made of a master dot pattern for photoetching a graded-hole shadow mask according to the invention will be described in some detail as follows.

I. Photographing with process camera.

A. Equipment and material.

1. Camera. A process camera is suitable for the purpose. 2. Light source.

A highly stable light source such as a 100 v., 500 w. tungsten projection lamp used for an ordinary camera may be successfully used. A housing for the lamp may incorporate a cooling fan, a heat-absorbing filter and a light-diffusing plate (opal glass). In front of this diffusing plate, there is installed a photographically or mechanically fabricated grading mask having a brightness greater at the center and progressively lower the farther away from the center of the mask.

3. Lens.

An ordinary process lens (such as for example Aponicol f=60 cm.) may be used with a round aperture waterhouse stop or an iris diaphragm.

4. Screen.

A 60 cross angle glass screen provided with a rhombic opening and having a width ratio of transparent spot to opaque spot of l:ll:2 is suitable. Alternatively, a white dot screen having a circular or regular polygonal transparent dot at each corner of a equilateral triangle may be fabricated from a dry plate or film. The screen is secured to a screen holder.

5. Photographic material.

This may be a dimensionally stable base Iith-type film or dry plate.

B. Process of preparing a pattern.

The apparatus required for the production of a master dot pattern comprises a light source 9, a heat-absorbing filter 20 and a light-diffusing plate 21 that are journaled in a lamp housing with a grading mask 22 placed in front of the diffusing plate 21, as shown in FIG. 6. Designated at 23 is a process camera which is mounted before the grading mask. Designated at 24 is a camera lens and at 25 is an iris diaphragm. The screen in the screen holder is placed before the camera. Designated at 26 is a photographic material (dry plate or film).

An example of the above equipment layout shows the I glass screen having a white-black ratio of 1:2 and a line pitch of 0.51 m/m is placed 35 mm. from the ground glass 24' of the camera 23. Light source 9 may be a 100 v., 500 w. projection lamp which gives 90 seconds of exposure of light to a lith-type film 26 in a vacuum bag while the lens iris is set at f:64. The film thereby exposed may be developed using Kodaris Super developer with care not to cause irregularities in the development.

ll. Photographing with printing frame.

A. Equipment and material.

1. Printing frame. This may be an ordinary vacuum suction type printer.

2. Light source.

As a light source, there may be used a tungsten lamp which may be located centrally at the bottom of the printing frame. Inserted in between the lamp and the top glass of the printer is an opal glass or suitable synthetic resin, upon which is placed a photographically or mechanically fabricated grading mask having a greater transparency at the center and progressively lower transparency the farther away from the center of the mask. in this manner, the intensity of light is reduced progressively from the center toward the periphery of the pattern.

3. Screen.

A 60 cross angle screen or a white dot screen prepared therefrom with use of a dry plate or film which has a transparent spot of any suitable configuration at each comer of an equilateral triangle may be used.

4. Photographic material.

A lith-type film or dry plate may be used as in the case of process I above.

B. Process of preparing a pattern.

The apparatus required for the production of a master dot pattern in this embodiment of the invention comprises a printing frame 27 having a top glass 28 upon which is placed a screen 29 with emulsified face up and a spacer of transparent sheet 30 placed on the screen. Alternatively, the screen 29 may be placed so that the emulsified surface thereof firmly contacts the top glass 28 of the printing frame thereby spacing the screen 29 uniformly apart from the emulsified face of the photographic material 31 and thereby enabling the adjustment of screen dot printing. The distance between the light source 32 and the printer top glass 28 is held at about 50-100 cm. A diffusing plate 33 made of opal glass or synthetic resin is inserted in such a manner as to permit the distance (height) to be adjusted, and thus the grading of screen dots may be varied at will. Placed on the opal diffusing plate 33 is a grading mask 35 which is covered at the periphery with a sheet of black paper 34. The grading mask 35 may preferably be of a disc form which has a diameter sufficiently larger to cover the area required for exposure. The rest of the process involved in the printing of a master dot pattern is the same as described in process I hereinabove.

lll. Photographing with graded-hole contact screen.

in accordance with the above processes I and II, a low contrast photographic material is used in place of a lith-type photographic material with which to finish a contact screen which has an obscure dot edge when photographed. In this instance, the size of these obscure screen dots is reduced progressively from the center toward the peripheral portion of the screen by the effect of the grading mask. This contact screen is adhered to a lith-type film and exposed to light uniformly over, the entire surface, thereby obtaining a negative pattern having a clear'cut edge.

The screen dot pattern obtained from either of the above processes may be provided with suitable symbols including display line, a cutting line, codes, numbers, etc., before it is finished as a photoetching master pattern.

Having thus described the principal concepts of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made without limit to such specific embodiments as herein described and illustrated and without departing from the spirit and scope of the invention defined in thefollowing claims. For example, the pore distribution may be concentrically elliptical instead of concentrically circular. Or, some pores may be larger or smaller than the others.

What is claimed is:

1. A process for the production of a master dot pattern for photoetching of graded-hole shadow masks for use in color television picture tubes comprising mounting a grading mask in front of a light source, forming the grading mask with a concentrically and progressively decreasing transparency from the center thereof toward the periphery thereof, inserting a screen between said light source and a photographic material, subjecting said photographic material to exposure through said screen and mask and developing the exposed photographic material.

2. A process for the production of a master dot pattern for photoetching of graded-hole shadow masks for use in color television picture tubes comprising placing a screen upon a glass member on top of a printing frame, positioning a photographic material with an emulsified surface thereof above said screen, installing a grading mask between a light source and said screen, forming said mask with a concentrically and progressively decreasing transparency from the center thereof toward the periphery thereof, subjecting said photographic material to exposure of light through said mask and developing the exposed photographic material.

, 3. The process for the production of a master dot pattern for photoetching of graded-hole shadow masks according to claim 1 further comprising adhering a relatively high contrast photographic material to a contact screen made of a relatively low contrast photographic material, applying light uniformly over the entire surface thereof and printing same to produce a negative pattern having screen dots with clear-cut edges.

4. The process of claim 1 wherein said grading mask is formed with a light transmittance characteristic such that exposure light is greatest at the center of said grading mask and decreases in accordance with distance away from the center in a concentrically circular fashion.

5. The process of claim 1 wherein said grading mask is formed as a plate with a transparent portion and an opaque portion, said plate being held in opposition to the photographic material, and providing relative rotation between said plate and said photographic material.

6. The process of claim 3 wherein said grading mask is formed as an opaque disc with a transparent sector and comprising mounting an optical wedge having a variable density distribution in said sector, and providing relative rotation between said photographic material and said disc.

7. The process of claim 1 wherein said grading mask is formed by stacking a plurality of sheets of uniformly transparent material with circular windows which are concentrically arranged and successively increasing in diameter.

8. The process of claim 1 wherein said grading mask is formed of a semitransparent material in the form of a plate with a conical recess therein so that the thickness of said semitransparent material is progressively reduced from the center to the periphery thereof.

9. The process of claim 2 wherein said grading mask is formed as a plate with a transparent portion and an opaque portion, said plate being held in opposition to the photographic'material is a negativ e irnage forming photographic material.

12. A process as claimed in claim 2 wherein the photographic material is a negative-image forming photographic material. 

2. A process for the production of a master dot pattern for photoetching of graded-hole shadow masks for use in color television picture tubes comprising placing a screen upon a glass member on top of a printing frame, positioning a photographic material with an emulsified surface thereof above said screen, installing a grading mask between a light source and said screen, forming said mask with a concentrically and progressively decreasing transparency from the center thereof toward the periphery thereof, subjecting said photographic material to exposure of light through said mask and developing the exposed photographic material.
 3. The process for the production of a master dot pattern for photoetching of graded-hole shadow masks according to claim 1 further comprising adhering a relatively high contrast photographic material to a contact screen made of a relatively low contrast photographic material, applying light uniformly over the entire surface thereof and printing same to produce a negative pattern having screen dots with clear-cut edges.
 4. The process of claim 1 wherein said grading mask is formed with a light transmittance characteristic such that exposure light is greatest at the center of said grading mask and decreases in accordance with distance away from the center in a concentrically circular fashion.
 5. The process of claim 1 wherein said grading mask is formed as a plate with a transparent portion and an opaque portion, said plate being held in opposition to the photographic material, and providing relative rotation between said plate and said photographic material.
 6. The process of claim 3 wherein said grading mask is formed as an opaque disc with a transparent sector and comprising mounting an optical wedge having a variable density distribution in said sector, and providing relative rotation between said photographic material and said disc.
 7. The process of claim 1 wherein said grading mask is formed by stacking a plurality of sheets of uniformly transparent material with circular windows which are concentrically arranged and successively increasing in diameter.
 8. The process of claim 1 wherein said grading mask is formed of a semitransparent material in the form of a plate with a conical recess therein so that the thickness of said semitransparent material is progressively reduced from the center to the periphery thereof.
 9. The process of claim 2 wherein said grading mask is formed as a plate with a transparent portion and an opaque portion, said plate being held in opposition to the photographic material, and providing relative rotation between said plate and said photographic material.
 10. The process of claim 2 wherein said grading mask is formed by stacking a plurality of sheets of uniformly transparent material with circular windows which are concentrically arranged and successively increasing in diameter.
 11. A process as claimed in claim 1 wherein the photographic material is a negative-image forming photographic material.
 12. A process as claimed in claim 2 wherein the photographic material is a negative-image forming photographic material. 